The invention relates to a circuit arrangement controlling the speed of an electric motor.
In a known circuit of this type (DE-OS No. 2,906,388), the series connection of the armature winding of the electric motor and a controllable rectifier is connected between two field winding halves. Arranged in parallel with this series connection is a potentiometer whose pickup is connected to the inverting input of an operational amplifier configured as integrating amplifier. The inverting input of the operational amplifier is located at the terminal of the potentiometer connected directly to an electrode of the controllable rectifier. Arranged in parallel with the field winding portion starting at this reference potential point is a series connection of a capacitor and an ohmic resistor, with the capacitor being connected to the reference potential point. A line leads from the point of connection of the capacitor with the resistor to the inverting input of the operational amplifier. This generates a signal which is proportional to the motor current and is switched in opposition to the signal picked up at the potentiometer. The output of the operational amplifier controls a phase control unit which, in turn, charges the control electrode of the controllable rectifier. A drawback of this circuit arrangement is that the potentiometer for setting the rpm also picks up the voltage drop that builds up during the idle period of the controllable rectifier. To suppress this part of the control voltage present at the potentiometer pickup, since it falsifies the measuring result, a special switch is necessary which short-circuits the control signal at the potentiometer pickup with respect to the reference potential point during the idle time of the controllable rectifier.